Cross-media systems, in which a variety of image apparatuses are connected to an open system, are becoming very popular in response to advances in digital image apparatuses and network technology centering on the Internet. In open systems, image apparatuses and applications need to have a common interface and establish a configuration with high versatility and extendibility. From the aspect of color reproduction, an image apparatus sending color information, i.e., a camera or scanner, needs to send accurate color information captured to the open system. On the other hand, an image apparatus receiving and displaying color information, i.e., a display or printer, needs to display accurately the color information received. For example, even though the camera captures accurate color information, the color reproducibility of the entire system is degraded if the display can only display color information inappropriately.
To solve the above point, the IEC (International Electro-technical Commission) has established a standard, sRGB, for standard displays. This standard clearly defines the relation between RGB video signals and colorimetric values by matching the chromaticity point of the three RGB primary colors to the colorimetric parameter defined in Rec. 709 as recommended by ITU-R (International Telecommunication Union Radio Communication). Accordingly, displays complying with this standard-display standard can calorimetrically display the same colors if the same RGB video signals are given. On the other hand, displays are often used for video editing as well as viewing the images displayed. For example, displays are used for creating originals for catalog prints. Therefore, “sRGB display,” the standard display, which allows colorimetrical control, is a key to color management including hard copying such as printing.
However, the above conventional image display device has the following disadvantages. “Pointer color gamut” and “SOCS color gamut” are databases that contain the color distribution of typical reflective objects in the natural world. These databases give a dynamic range of colorimetric value input to cameras, and also provide design references for the color gamut of displays. In other words, the color gamut covering at least the sum of “Pointer color gamut” and “SOCS color gamut” [hereafter (Pointer+SOCS) gamut] is required for accurately displaying the colors of naturally reflective objects.
FIG. 3A is a sectional view of a gamut solid in the CIELAB space and shows the relation between the color gamut of the sRGB display and the (Pointer+SOCS) color gamut, which is the database for color distribution of naturally reflective objects, on the plane a*-b* at equal luminance L*=50. It is apparent from FIG. 3 that color gamut 2001 of the sRGB display is smaller than color gamut 2002 of (Pointer+SOCS), indicating that the sRGB display cannot display certain naturally reflective objects. Calculation of gamut volume in the CIELAB space reveals that the sRGB display covers about 76% of the (Pointer+SOCS) color gamut, and thus 24% of the (Pointer+SOCS) color gamut is not displayable on the sRGB display. Accordingly, even though the camera captures a precise image securing a wide dynamic range covering the color distribution of a naturally reflective object, about 24% of the precisely captured image is not displayable on the sRGB display.
A conventional image device solving this disadvantage is disclosed, for example, in the Japanese Patent Laid-open Application No. 2001-306023. FIG. 10 shows the conventional image display device disclosed in this laid-open patent.
In FIG. 10, the image display device configures multiple pixels 36 aligned in a matrix. These pixels consists of sub-pixel 36R for red light, sub-pixel 36G for green light, sub-pixel 36B for blue light, and sub-pixel 36C emitting light of cyan, magenta, or yellow. This sub-pixel 36C is specified as a point on the chromaticity diagram outside of a triangular region formed by linking points of red (R), green (G), and blue (B) on the chromaticity diagram shown in FIG. 11. The CMY in FIG. 11 indicates cyan (C), magenta (M), and yellow (Y).
In the above conventional configuration, however, a color display range changes with luminance because no restriction on luminance is provided. This makes it difficult to secure compatibility with the sRGB display when the fourth primary color is added.
More specifically, the shape and size of the color gamut of the display are determined by the positions of the primary color points. Since the color space is three-dimensional, primary color points have three-dimensional coordinates. In the case of the sRGB display, each primary color R (primary color red), primary color G (primary color green), and primary color B (primary color blue) possesses two-dimensional chromaticity coordinates (x, y) and one-dimensional luminance Y The (Pointer+SOCS) color gamut is also a three-dimensional solid. In order to display the precise colors of naturally reflective objects, two-dimensional chromaticity coordinates and one-dimensional luminance of primary colors of the display need to be determined such that the color gamut solid of the display covers the (Pointer+SOCS) color gamut solid to the maximum extent possible.